Formal synthesis of (±)-methyl rocaglate using an unprecedented acetyl bromide mediated Nazarov reaction

Lewis Base-Catalyzed Intramolecular Vinylogous Aldol Reaction and Chemoselective Syntheses of 3-Hydroxy-2,3-Disubstituted Dihydrobenzofurans and Indolines

A Lewis base-catalyzed intramolecular vinylogous aldol reaction of o-(allyloxy)phenyl ketoesters or o-(allylamino)phenyl ketoesters has been developed. This reaction provides ready access to 3-hydroxy-2,3-disubstituted dihydrobenzofurans and indolines in high yields with excellent chemoselectivity and diastereoselectivity. An acid-promoted dehydration of such products further extends the utility of the reaction to the synthesis of 2-alkenyl benzofurans and indoles.

Merging Strategy, Improvisation, and Conversation to Solve Problems in Target Synthesis

Total synthesis has long been depicted as the quest to conquer the structures created by nature, requiring an unflinching, single-minded devotion to the task. The goal is achieved by chemists with grit, strength of will, and a competitive spirit. While there is some truth to this viewpoint, it does not fully capture the rich experiences gained in this research realm. In our lab, strategic planning, improvisation, and conversation have worked in concert to enable progress. This Account summarizes our efforts to synthesize four different bioactive targets: merrilactone A, rocaglamide, phomactin A, and tetrapetalone A. Certain missteps were integral to success in these synthetic projects. As such, we include the hiccups, and their roles in the evolution of the strategies, along with the results that aligned with our expectations.Two of these projects (merrilactone A and rocaglamide) culminated in the total synthesis of the targets. The challenges presented by merrilactone A spawned a new design strategy for pentannulation using Nazarov cyclization chemistry. This work demonstrated that Lewis acid catalysis is often a viable electrocyclization strategy in activated, polarized pentadienyl cation intermediates. We sought to apply the same logic to the rocaglamide target, but precursors we prepared did not behave according to plan. This situation pushed us to adapt our approach to match the innate reactivity of the substrate, resulting in an on-the-spot improvisation that was not only integral to the success of the project but also expanded our understanding of pentadienyl cation chemistry. In the other two projects (phomactin A and tetrapetalone A), we did not complete a total synthesis but did build the polycyclic core of the target. Even though the hetero [4 + 2] cycloaddition plan for assembling the macrocyclic oxadecalin ring system of phomactin A failed, the original experimental design still enabled us to solve the problem. Through a wholly unanticipated series of events, our focus on the oxadecalin ring system primed us for the discovery of a sequential iodoaldol/oxa-Michael sequence, using the original [4 + 2] building blocks. Then, the bridging ring present in phomactin A demanded we implement this sequence in a transannular fashion. Finally, our successful synthesis of the tetrapetalone core was enabled by consultations with others in the community. Each bond formation seemed to require different expertise, and in three separate instances (C-N cross-coupling, diastereoselective ring-closing metathesis, and oxidative dearomatization) synthetic challenges were overcome through conversation and collaboration.In our experience, the amount of creative power we summon during a target synthesis project correlates directly with the magnitude of the structural challenges we face. When reactivity surprises us, we analyze the problem anew, consult with colleagues, and improvise with the tools at hand. The inevitable misbehavior of a complex system is a strong motivating force, and one that has helped to shape our research program for nearly two decades.

Intercepted Retro-Nazarov Reaction: Syntheses of Amidino-Rocaglate Derivatives and Their Biological Evaluation as eIF4A Inhibitors

Rocaglates are a family of natural products isolated from the genus Aglaia which possess a highly substituted cyclopenta[b]benzofuran skeleton and inhibit cap-dependent protein synthesis. Rocaglates are attractive compounds due to their potential for inhibiting tumor cell maintenance in vivo by specifically targeting eukaryotic initiation factor 4A (eIF4A) and interfering with recruitment of ribosomes to mRNA. In this paper, we describe an intercepted retro-Nazarov reaction utilizing intramolecular tosyl migration to generate a reactive oxyallyl cation on the rocaglate skeleton. Trapping of the oxyallyl cation with a diverse range of nucleophiles has been used to generate over 50 novel amidino-rocaglate (ADR) and amino-rocaglate derivatives. Subsequently, these derivatives were evaluated for their ability to inhibit cap-dependent protein synthesis where they were found to outperform previous lead compounds including the rocaglate hydroxamate CR-1-31-B.

Chemical Synthesis Enables Structural Reengineering of Aglaroxin C Leading to Inhibition Bias for Hepatitis C Viral Infection

As a unique rocaglate (flavagline) natural product, aglaroxin C displays intriguing biological activity by inhibiting hepatitis C viral entry. To further elucidate structure-activity relationships and diversify the pyrimidinone scaffold, we report a concise synthesis of aglaroxin C utilizing a highly regioselective pyrimidinone condensation. We have prepared more than 40 aglaroxin C analogues utilizing various amidine condensation partners. Through biological evaluation of analogues, we have discovered two lead compounds, CMLD012043 and CMLD012044, which show preferential bias for the inhibition of hepatitis C viral entry vs translation inhibition. Overall, the study demonstrates the power of chemical synthesis to produce natural product variants with both target inhibition bias and improved therapeutic indexes.

De novo synthesis of benzofurans via trifluoroacetic acid catalyzed cyclization/oxidative aromatization cascade reaction of 2-hydroxy-1,4-diones

A simple metal-free cyclization/oxidative aromatization reaction starting from 2-hydroxy-1,4-diones for the de novo synthesis of benzofurans was developed.

Iodine-Catalyzed Iso-Nazarov Cyclization of Conjugated Dienals for the Synthesis of 2-Cyclopentenones

Molecular iodine was identified as an efficient catalyst for the cycloisomerization of conjugated dienals to substituted 2-cyclopentenones. DFT calculations suggested an unexpected concerted character for this cyclization.

Enantioselective synthesis of cyclopenta[b]benzofurans via an organocatalytic intramolecular double cyclization

An enantioselective organocatalytic strategy, combining Brønsted base and N-heterocyclic carbene catalysis in a unique manner, is demonstrated for a concise construction of the privileged cyclopenta[b]benzofuran scaffold, present in many bioactive compounds having both academic and commercial interests. The reaction concept relies on an intramolecular one-pot double cyclization involving a cycle-specific enantioselective Michael addition followed by a benzoin condensation of ortho-substituted cinnamaldehydes. Cyclopenta[b]benzofurans were achieved in moderate to good yields, with excellent stereoselectivities. A proof of principle for a diastereodivergent variation is demonstrated through the synthesis of cyclopenta[b]benzofurans containing two contiguous aromatic substituents in a substitution pattern present in commercial and natural compounds. Furthermore, several transformations have been performed, demonstrating the synthetic utility of the products. Finally, insights into the activation mode and stereoindution models are presented for this new synthetic strategy.

Nazarov reaction: current trends and recent advances in the synthesis of natural compounds and their analogs

Recent applications of the Nazarov reaction to stereoselective synthesis of pharmacology-relevant cyclopentenone scaffolds are summarized.

The iso-Nazarov reaction

The acid-promoted cycloisomerization of conjugated dienals and linearly-conjugated dienones for the synthesis of five-membered ring systems is reviewed.

Cytotoxic Rocaglate Derivatives from Leaves of Aglaia perviridis

Rocaglates are a series of structurally complex secondary metabolites with considerable cytotoxicity that have been isolated from plants of the Aglaia genus (Meliaceae). A new rocaglate (aglapervirisin A, 1) and its eight new biosynthetic precursors of rocaglate (aglapervirisins B-J, 2-9) together with five known compounds, were isolated from the leaves of Aglaia perviridis. Their structures were elucidated based on a joint effort of spectroscopic methods [IR, UV, MS, ECD, 1D- and 2D-NMR, HRESIMS], chemical conversion and single-crystal X-ray diffraction. Among these isolates, three (1, 10-11) were silvestrols, a rare subtype rocaglates, exhibiting notable cytotoxicity against four human tumor cell lines, with IC50 values between 8.0 and 15.0 nM. Aglapervirisin A (1) induces cell cycle arrest at the G2/M-phase boundary at concentration 10 nM accompanied by reductions in the expression levels of Cdc2 and Cdc25C in HepG2 cells after 72h co-incubation, and further induces the apoptosis of HepG2 cells at concentrations over 160 nM.

Total synthesis of a biotinylated rocaglate: Selective targeting of the translation factors eIF4AI/II

The total synthesis of a biotinylated derivative of methyl rocaglate is described. This compound was accessed from synthetic methyl rocaglate (2) via formation of the propargyl amide and subsequent click reaction with a biotin azide. Affinity purification revealed that biotinylated rocaglate (8) and methyl rocaglate (2) bind with high specificity to translation factors eIF4AI/II. This remarkable selectivity is in line with that found for the more complex rocaglate silvestrol (3).

4π electrocyclisation in domino processes: contemporary trends and synthetic applications towards natural products

Recent most instructive and reliable literature reports which deal with domino processes involving conrotatory 4π electrocyclic reactions, along with a precise mechanistic insight and latest synthetic applications towards biologically active natural products are concisely reviewed. To inspire further research in this domain, a real insight into the overarching emerging themes and potential imminent prospects is also provided.

Synthesis of each enantiomer of rocaglamide by means of a palladium(0)-catalyzed Nazarov-type cyclization

A recently reported Pd(0)-catalyzed asymmetric Nazarov-type cyclization has been successfully applied in the key step of the first catalytic asymmetric total synthesis of (-)-rocaglamide (natural) and (+)-rocaglamide. The stereochemistry at the C3 position that controls the stereochemistry of all other stereocenters is determined in the cyclization step. This versatile and modular synthesis proceeds from simple reagents.

New anti-angiogenic leading structure discovered in the fruit of Cimicifuga yunnanensis

Cimyunnins A–C (1–3), characterized with an unusual fused cyclopentenone ring G, together with cimyunnin D (4), possessing a highly rearranged γ-lactone ring F, were characterized from the fruit of Cimicifuga yunnanensis. Their structures were elucidated by spectroscopic analysis, X-ray diffraction, and density functional theory calculations. In addition, cimyunnin A exhibited comparable anti-angiogenic activities to those of sunitinib, a clinically-used first-line angiogenesis inhibitor, in the in vitro and ex vivo studies.

Efficient catalytic enantioselective Nazarov cyclizations of divinyl ketoesters

An efficient catalytic enantioselective Nazarov cyclization of divinyl ketoesters is developed using a BOX/Cu(ii) catalyst, affording various optically active cyclopent-2-enone esters.

Rocaglamide, silvestrol and structurally related bioactive compounds from Aglaia species

Covering: 2006 to 2013. Investigations on the chemistry and biology of rocaglamide, silvestrol and structurally related bioactive compounds from Aglaia species during the period 2006-2013 are reviewed. Included are new phytochemical studies of naturally occurring rocaglamide derivatives, an update on synthetic methods for cyclopenta[b]benzofurans, and a description of the recent biological evaluation and mechanism-of-action studies on compounds of this type.

Flavaglines: potent anticancer drugs that target prohibitins and the helicase eIF4A

Flavaglines are complex natural products that are found in several medicinal plants of Southeast Asia in the genus Aglaia; these compounds have shown exceptional anticancer and cytoprotective activities. This review describes the significance of flavaglines as a new class of pharmacological agents and presents recent developments in their synthesis, structure–activity relationships, identification of their molecular targets and modes of action. Flavaglines display a unique profile of anticancer activities that are mediated by two classes of unrelated proteins: prohibitins and the translation initiation factor eIF4A. The identification of these molecular targets is expected to accelerate advancement toward clinical studies. The selectivity of cytotoxicity towards cancer cells has been shown to be due to an inhibition of the transcription factor HSF1 and an upregulation of the tumor suppressor TXNIP. In addition, flavaglines display potent anti-inflammatory, cardioprotective and neuroprotective activities; however, the mechanisms underlying these activities are yet to be elucidated.

Enantioselective photocycloaddition of 3-hydroxyflavones: total syntheses and absolute configuration assignments of (+)-ponapensin and (+)-elliptifoline

We have previously reported development of biomimetic, asymmetric [3 + 2] photocycloadditions between 3-hydroxyflavones and cinnamate dipolarophiles to access (-)-rocaglamide and related natural products. Herein, we describe enantioselective syntheses of aglain cycloadducts leading to the first total syntheses and absolute configuration assignments of the aglain natural products (+)-ponapensin and (+)-elliptifoline.